EP2570869A1 - Timepiece with oscillators coupled in chronograph mode - Google Patents

Abstract

The timepiece (1) has an oscillator (15) oscillating at a frequency and connected by a gear train (5) to an energy source (9) to display the time. A chronograph system (51) comprises another gear train (25) that is connected to the former gear train via a coupling device, and another oscillator (35) that is connected to the latter gear train and oscillates at another frequency. The latter gear train is connected to the former gear train by an elastic coupling unit (41) to synchronize the rate of the oscillators using the energy source when the coupling unit authorizes time measurement.

Description

Field of the invention

The invention relates to a timepiece with oscillators coupled in chronograph mode and such a timepiece comprising two oscillators intended to display at least one value less than or equal to the second with a better resolution and / or a better accuracy .

Background of the invention

It is known to train timepieces whose frequency is increased to improve the resolution. However, these timepieces can be very sensitive to shocks or very greedy energy which makes them marginal.

It is therefore clear that it is easier to manufacture a template by mounting a low frequency oscillator, typically 4 Hz, to display the time and another high frequency oscillator, typically 10 or 50 Hz, independent of the first to display a measured time with better resolution. However, after several seconds, it becomes apparent that the display of the seconds of each oscillator is no longer the same, which can raise questions about the quality of the timepiece.

Summary of the invention

The object of the present invention is to overcome all or part of the disadvantages mentioned above by proposing a timepiece capable of displaying the time or time measured with a system. Chronograph with better resolution while maintaining the usual robustness for a mechanical watch, reduced power consumption and minimal drift between the time display and the measured time display even if the latter is greater than one minute .

For this purpose, the invention relates to a timepiece comprising a first oscillator oscillating at a first frequency and connected by a first gear to a power source to display the time and a chronograph system having a second gear connected the first gear via a clutch device for selectively measuring a time characterized in that the chronograph system further comprises a second oscillator connected to the second train which oscillates at a second frequency and in that the second wheel is connected to the first gear by elastic coupling means for synchronizing the operation of the two oscillators with the same energy source when the clutch device allows said measurement of a time.

It is therefore understandable that, even in the event of shocks, the variations in operation will be minimal thanks to the construction allowing the synchronization of the two oscillators. Therefore, the timepiece according to the invention is capable of displaying the time, and / or the time measured with a chronograph system, with a better resolution and / or a better accuracy while guaranteeing a great robustness, a low consumption and a slight drift between the display of the time and the display of the measured time even if the latter is greater than one minute.

• les moyens de couplage élastique sont formés par un ressort reliant une roue du premier rouage avec une autre du deuxième rouage ;
• les moyens de couplage élastique relient les roues des secondes respectivement du premier rouage et du deuxième rouage ;
• le premier oscillateur reçoit le plus de couple de la source d'énergie et, préférentiellement, au moins 75% du couple ;
• le premier oscillateur possède un isochronisme de meilleure qualité que le deuxième oscillateur afin de faciliter la synchronisation de ce dernier ;
• le premier oscillateur comporte un facteur de qualité supérieur à celui du deuxième oscillateur ;
• le deuxième oscillateur comporte un facteur de qualité inférieur à 100 afin d'obtenir une stabilisation plus rapide ;
• selon un premier mode de réalisation, les première et deuxième fréquences sont identiques ;
• les deux fréquences sont supérieure à 5 Hz pour afficher avec une meilleure résolution et/ou une meilleure précision aussi bien l'heure que le temps mesuré ;
• selon un deuxième mode de réalisation, la première fréquence est plus élevée que la deuxième fréquence afin d'afficher l'heure avec une meilleure résolution et/ou une meilleure précision ;
• la première fréquence est au moins égale à 10 Hz et la deuxième fréquence entre 1 et 5 Hz ;
• selon un troisième mode de réalisation, la première fréquence est plus basse que la deuxième fréquence afin d'afficher le temps mesuré avec une meilleure résolution et/ou une meilleure précision ;
• la deuxième fréquence est au moins égale à 10 Hz et la première fréquence entre 3 et 5 Hz.

According to other advantageous features of the invention:

• the elastic coupling means are formed by a spring connecting a wheel of the first wheel with another of the second wheel;
• the elastic coupling means connect the wheels of the seconds respectively of the first wheel and the second wheel;
• the first oscillator receives the most torque from the energy source and, preferably, at least 75% of the torque;
• the first oscillator has an isochronism of better quality than the second oscillator to facilitate the synchronization of the latter;
• the first oscillator has a higher quality factor than the second oscillator;
• the second oscillator has a quality factor of less than 100 in order to obtain a faster stabilization;
• according to a first embodiment, the first and second frequencies are identical;
• the two frequencies are greater than 5 Hz to display with a better resolution and / or a better accuracy both the time and the measured time;
• according to a second embodiment, the first frequency is higher than the second frequency in order to display the time with a better resolution and / or a better accuracy;
• the first frequency is at least 10 Hz and the second frequency is between 1 and 5 Hz;
• according to a third embodiment, the first frequency is lower than the second frequency in order to display the measured time with a better resolution and / or a better accuracy;
• the second frequency is at least 10 Hz and the first frequency is between 3 and 5 Hz.

Brief description of the drawings

• la figure 1 est un exemple de pièce d'horlogerie selon l'invention ;
• la figure 2 est un exemple de moyens de couplage élastique selon l'invention ;
• les figures 3 et 4 sont des simulations de synchronisation pour deux exemples de pièces d'horlogerie selon l'invention.

Other particularities and advantages will emerge clearly from the description which is given hereinafter, by way of indication and in no way limiting, with reference to the appended drawings, in which:

• the figure 1 is an example of a timepiece according to the invention;
• the figure 2 is an example of elastic coupling means according to the invention;
• the Figures 3 and 4 are synchronization simulations for two examples of timepieces according to the invention.

Detailed Description of the Preferred Embodiments

As illustrated in figures 1 and 2 the invention relates to a timepiece 1 comprising a first resonator 3 and connected by a first gear 5 via a first escapement 7 to an energy source 9. The first resonator 3 and the first escapement 7 thus form a first oscillator 15 oscillating at a first frequency ƒ ₁ to display the time. The timepiece 1 also comprises a chronograph system 51 comprising a second gear 25 connected to the first gear 5 via a clutch device 44 for selectively measuring a time.

Advantageously according to the invention, the chronograph system 51 further comprises a second oscillator 35 connected to the second gear 25 and which oscillates at a second frequency ƒ ₂ . In addition, according to the invention, the second gear train 25 is advantageously connected to the first gear train 5 by elastic coupling means 41 in order to synchronize the operation of the two oscillators 15, 35 with the aid of the same source. energy 9 when the clutch device 44 allows said measurement of a time.

As seen in the example of the figure 1 , the energy source 9 is preferably a barrel, that is to say a source of mechanical energy accumulation. In addition, in the same figure, the second oscillator 35 comprises a second resonator 23 connected to the second gear 25 via a second escapement 27.

Preferably, according to the invention, the elastic coupling means 41 are formed by a spring 43 connecting a wheel of the first gear train 5 with another of the second gear train 25. As illustrated in FIG. figure 2 , the elastic coupling means 41 connect, preferably according to the invention, the second wheels respectively of the first gear 5 and the second gear 25 when the clutch device 44 is in its coupled position, that is to say that it allows the total transmission of the torque it receives.

Preferably according to the invention, it can be seen that a double wheel 42 is used. As better visible at the figure 2 , it is formed by a first plate 45 connected via a reference 46 of the clutch device 44 to the first gear 5. The double wheel 42 further comprises a second plate 47 connected directly or indirectly to the second wheel 25 of the chronograph system 51. two boards 45, 47 are integral with an axle 48 respectively in a crazy manner and in a fixed manner. Finally, the spring 43 of the elastic coupling means 41 is preferably mounted between the fastener 49 fixed on the stretcher of the board 45 and the flange 50 of the axis 48. It is therefore understood that the boards 45 and 47, and incidentally, the workings 5 and 25 may be angularly offset by the elastic coupling of the spring 43 when the clutch device 44 is in its coupled position.

Preferably according to the invention, the time display, that is to say the hours, the minutes and possibly the seconds, is made from the first wheel 5. While the display of the measured time by the chronograph system 51 is, preferably, made from the second wheel 25.

Depending on the desired application for the timepiece, the first ƒ ₁ and second ƒ ₂ frequencies are identical or not. Thus, in a first embodiment, the first and second frequencies ƒ ₁ , ƒ ₂ are identical and preferably greater than 5 Hz to display with a better resolution and / or a better accuracy both the time and the measured time. In such an embodiment, the frequencies ƒ ₁ , ƒ ₂ may, for example, be equal to 10 Hz or 50 Hz to respectively display 1/20 or 1/100 of seconds.

In a second embodiment, the first frequency f ₁ is higher than the second frequency f _{2 in} order to display the time with a better resolution and / or a better accuracy. Similarly to the first embodiment, the first frequency f ₁ is at least 10 Hz and the second frequency f ₂ is preferably between 1 and 5 Hz. By way of example, it may be desired that the second of the measured time is incremented by a single step per second, that is to say that the second frequency f ₂ is equal to 1 Hz, "in the manner" of a quartz watch.

In a third embodiment, the first frequency f ₁ is lower than the second frequency f _{2 in} order to display the measured time with better resolution and / or better accuracy. In this embodiment, inverse to the second embodiment, the second frequency f ₂ is at least 10 Hz and the first frequency f ₁ is preferably between 3 and 5 Hz.

Simulations have been developed below to describe the synchronization between these two oscillators 15, 35. Arbitrarily, the third embodiment has been chosen for the explanation. Thus, the oscillator 15 is selected from the low frequency type and called the first oscillator. In fact, in the example below, the second oscillator will be the oscillator 35 of the high frequency type which will synchronize with the low frequency oscillator.

à laquelle la marche est nulle. De plus, on suppose que le premier oscillateur 15 a toujours une marche sensiblement nulle en faisant faiblement varier son amplitude.Preferably, according to the invention, the second oscillator 35 is chosen with a strong anisochronism as a function of the amplitude, described by the slope of anisochronism Γ , as well as by the amplitude ${\mathrm{AT}}_2^0$

at which the march is null. In addition, it is assumed that the first oscillator 15 always has a substantially zero step by slightly varying its amplitude.

The simulations show the evolution of the two oscillators 15, 35, that is to say their amplitudes and their phase shift state over time and thus make it possible to check the possibility of synchronization or not of the second oscillator 35 on the first oscillator 15.

, positive lorsqu'il oscille à une amplitude supérieure à $A_2^0$

et négative lorsqu'il oscille à une amplitude inférieure à $A_2^0$

.Preferably, the second oscillator 35 is constructed so that its march is zero when it oscillates at an amplitude ${\mathrm{AT}}_{}^{}$${}_2^0$

, positive when it oscillates at a greater amplitude than ${\mathrm{<figure-callout\; id="2"\; label="AT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">AT</figure-callout>}}_2^0$

and negative when it oscillates at a lower amplitude than ${\mathrm{<figure-callout\; id="2"\; label="AT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">AT</figure-callout>}}_2^0$

.

On the other hand, the elastic coupling means 41 are constructed so that the torque transmitted to the second wheel 25 remains constant if the two wheels 5, 25 rotate at the same speed, decreases if the second wheel 25 advances faster than the first wheel 5 (the spring 43 disarms) and increases if the second gear 25 advances less rapidly than the first gear 5 (the spring 43 is armed).

et dans laquelle le ressort 43 transmet, au deuxième rouage 25, le couple M ₂ nécessaire à maintenir le deuxième oscillateur 35 à l'amplitude $A_2^0$

.If the above conditions are satisfied, the timepiece will always evolve towards the stable situation where the second oscillator 35 oscillates at the amplitude ${\mathrm{AT}}_2^0$

and in which the spring 43 transmits, to the second wheel 25, the torque M ₂ necessary to maintain the second oscillator 35 at the amplitude ${\mathrm{AT}}_2^0$

.

. Comme expliqué ci-dessus, sa marche devient négative, c'est-à-dire que le deuxième oscillateur 35 prend du retard par rapport au premier oscillateur 5.Therefore, if the second oscillator 35 receives a torque less than M ₂ , its amplitude decreases, that is to say has a smaller amplitude than ${\mathrm{<figure-callout\; id="2"\; label="AT"\; filenames="imgf0003.png"\; state="\{\{state\}\}">AT</figure-callout>}}_2^0$

. As explained above, its operation becomes negative, that is to say that the second oscillator 35 is delayed relative to the first oscillator 5.

.It is therefore understood that the second wheel 25 will rotate more slowly than the first wheel 5 by arming the coupling spring 43, that is to say by increasing the torque transmitted to the second wheel 25. Therefore, the increasing torque, the amplitude of the second oscillator 25 is corrected automatically. It is therefore noted that both the torque and the amplitude of the second oscillator 35 synchronize structurally with the stable torque M ₂ and the stable amplitude ${\mathrm{AT}}_2^0$

.

, ce qui signifie que la marche du deuxième oscillateur 35 sera positive. Le deuxième rouage 25 prend donc de l'avance sur le premier rouage 5 en désarmant le ressort 43. Par conséquent, le couple sur le deuxième rouage 25 va diminuer vers le couple stable M ₂ et, l'amplitude du deuxième oscillateur 35, tendre à nouveau vers l'amplitude stable $A_2^0$

.Similarly, if the received torque exceeds the torque M ₂ then the amplitude of the second oscillator 35 becomes greater than the value ${\mathrm{AT}}_2^0$

which means that the step of the second oscillator 35 will be positive. The second gear 25 is therefore ahead of the first gear 5 by disarming the spring 43. Therefore, the torque on the second gear 25 will decrease towards the stable torque M ₂ and, the amplitude of the second oscillator 35, soft again towards stable amplitude ${\mathrm{AT}}_2^0$

.

.So we see that whatever the situation in which we are, either at the start of the watch or after a shock, the system will always evolve to stabilize on the stable situation where the torque on the second wheel 25 is M ₂ and the amplitude of the second oscillator 35 is ${\mathrm{AT}}_2^0$

.

• la « taille » des deux oscillateurs 15, 35 (par exemple les inerties I ₁ , I₂ si les résonateurs 3, 23 sont du type balancier ― spiral) ;
• les facteurs de qualité des deux oscillateurs 15, 35 : Q₁, Q₂ (qui est fonction de la taille de l'oscillateur) ;
• la pente d'anisochronisme du deuxième oscillateur : Γ ;
• l'amplitude du deuxième oscillateur pour laquelle sa marche est nulle : $A_2^0$
  
  ;
• le couple M ₂ du ressort 43 ;
• la rigidité angulaire K du ressort 43.

In a preferred manner according to the invention, it is assumed that the torque of the barrel 9 and the frequency f ₁ , f ₂ of the two oscillators 15, 35 are given parameters. We therefore understand that the parameters still to choose are:

• the "size" of the two oscillators 15, 35 (for example the inertias I ₁ , I ₂ if the resonators 3, 23 are of the balance-spring type);
• the quality factors of the two oscillators 15, 35: Q ₁ , Q ₂ (which is a function of the size of the oscillator);
• the anisochronism slope of the second oscillator: Γ ;
• the amplitude of the second oscillator for which its progress is null: ${\mathrm{AT}}_2^0$
  
  ;
• the torque M ₂ of the spring 43;
• the angular rigidity K of the spring 43.

• fraction du couple total que l'on souhaite transmettre au deuxième oscillateur, ce qui donne la valeur du couple M ₂. Selon l'invention, le premier oscillateur 15, reçoit le plus de couple par la source d'énergie 9 et, préférentiellement, au moins 75%.
• amplitude $A_{}^{}$${}_2^0$
  
  du deuxième oscillateur à laquelle on veut qu'il se stabilise (il faudra donc construire le deuxième oscillateur de façon à ce que sa marche soit sensiblement nulle à cette amplitude) ;
• taille du deuxième oscillateur (par exemple son inertie) pour que l'amplitude de stabilisation soit $A_{}^{}$${}_2^0$
  
  lorsqu'il reçoit le couple M ₂ (par l'intermédiaire du facteur de qualité) ;
• taille du premier oscillateur (par exemple son inertie) pour que l'amplitude de stabilisation soit acceptable (par l'intermédiaire du facteur de qualité) ;
• pente d'anisochronisme Γ du deuxième oscillateur 35 ;
• rigidité K du ressort 43.

Preferably, according to the invention, the parameters are chosen as follows:

• fraction of the total torque that it is desired to transmit to the second oscillator, which gives the value of the torque M ₂ . According to the invention, the first oscillator 15, receives the most torque by the energy source 9 and, preferably, at least 75%.
• amplitude ${\mathrm{AT}}_2^0$
  
  the second oscillator at which one wants it to stabilize (it will be necessary to build the second oscillator so that its march is substantially zero at this amplitude);
• size of the second oscillator (for example its inertia) so that the amplitude of stabilization is ${\mathrm{AT}}_2^0$
  
  when he receives the torque M ₂ (via the quality factor);
• size of the first oscillator (for example its inertia) so that the stabilization amplitude is acceptable (via the quality factor);
• anisochronism slope Γ of the second oscillator 35;
• stiffness K of the spring 43.

• le couple transmis au rouage 25 ne devienne jamais nul ;
• la marche du deuxième oscillateur 35 reste proche de sa fréquence zéro ;
• l'écart d'état entre les deux oscillateurs 15, 35 soit faible au « démarrage » ;
• le temps de stabilisation soit suffisamment court.

Advantageously according to the invention, it is also preferred to "adjust" K and Γ so that:

• the torque transmitted to the wheel 25 never becomes zero;
• the step of the second oscillator 35 remains close to its zero frequency;
• the difference in state between the two oscillators 15, 35 is weak at "start";
• the stabilization time is sufficiently short.

Empirically, it has been shown that it is preferable that the product K. Γ be kept identical to have the same stabilization time in the continuous approximation. Thus, increasing K (and thus decreasing Γ all the same) makes it possible to reduce the amplitude and torque fluctuations (thus preventing the pair from being canceled). On the other hand, it also increases the maximum state difference before stabilization, as well as instantaneous walking, which can become extreme. We must therefore find a compromise between these two effects.

It has also been found that increasing the frequency of the synchronizing oscillator (above the second oscillator 35) makes it possible to decrease the stabilization time. Finally, during the tests, it has been shown that reducing the quality factor of the oscillator that synchronizes (above the second oscillator) also makes it possible to reduce the stabilization time.

The Figures 3 and 4 are simulations performed as an example of execution. In the figure 3 , f ₁ = 4 Hz, f ₂ = 10 Hz, Q 1 = 200, Q ₂ = 50 and, in the figure 4 , f 1 = 4 Hz, f ₂ = 50 Hz, Q ₁ = 200, Q ₂ = 50 with for each simulation a product K.Γ identical.

choisi du deuxième oscillateur est d'environ ⅓. Ainsi au bout de respectivement 2 et 1,5 secondes, chaque oscillateur se stabilise à son amplitude synchronisée.Part A of each figure corresponds to the amplitude fraction of each oscillator relative to the reference amplitude if it received the entire torque of the energy source. We note that for the examples of the figures the amplitude ${\mathrm{AT}}_2^0$

chosen from the second oscillator is about ⅓. Thus after 2 and 1.5 seconds respectively, each oscillator stabilizes at its synchronized amplitude.

Part B of each figure corresponds to the fraction of torque that each oscillator receives from the energy source. Note that for the examples of the figures the proportion of torque chosen for the second oscillator is about 10%. Thus at the end of respectively 2 and 1.5 seconds, each oscillator receives in a stabilized manner its proportion of torque.

Part C of each figure corresponds to the operation of the second oscillator. We note that after 5.5 and 2 seconds respectively, the second oscillator stabilizes around its zero step.

Finally, the part D of each figure corresponds to the difference of state in seconds between each oscillator. We note that after 5 and 2 seconds respectively, the difference stabilizes at its zero value.

In view of the AD parts of Figures 3 and 4 , we thus perfectly find the conclusions stated above. It is therefore understandable that even in the event of shocks, the variations of step will be minimal thanks to the construction allowing the synchronization of the two oscillators. By Therefore, the timepiece according to the invention is capable of displaying the time, and / or the time measured with a chronograph system, with a better resolution and / or a better accuracy while guaranteeing a great robustness, a consumption low and a minimal drift between the display of the time and the display of the measured time even if the latter is greater than one minute.

Moreover, during the tests, it has been found that in addition to the fact that the first oscillator preferably has an isochronism of better quality than the second oscillator in order to facilitate synchronization of the latter, the second oscillator preferably comprises a quality factor lower than that of the first oscillator and, preferably, less than 100 to obtain a faster stabilization, that is to say typically less than 2 seconds.

Of course, the present invention is not limited to the illustrated example but is susceptible of various variations and modifications that will occur to those skilled in the art. In particular, a second clutch can be mounted on the wheel hours of the first gear 5 to avoid the addition of gearing in the second gear 25 to display the hours of the measured time. It is therefore understood that the second clutch would belong to the clutch device 44 and would trigger at the same time. Advantageously according to the invention, the two oscillators being synchronized, the display of the hours would increment also in a synchronized manner.

In addition, one may be in the first or second embodiment without the conclusions relating to the third embodiment differ. Thus, unlike the example above, if the first oscillator is of the high frequency type, the display of the time could be limited to hours and minutes from the first gear 5 in order to limit the propagation of couples induced by any shock at the high frequency oscillator. A second would then be displayed preferentially only on the second wheel 25.

In addition, when the first and / or second oscillator is of the high frequency type, that is to say greater than or equal to 5 Hz, a Clifford type oscillator can be used (see for example the document CH386344 incorporated by reference in this document). While when they have a frequency between 1 and 5 Hz, they will preferably be of the balance spring - spiral and Swiss lever escapement.

Of course, the elastic coupling means can not be limited to a double wheel 42 cooperating with a spring 43 as illustrated in FIGS. figures 1 and 2 . Other elastic coupling means can be envisaged, for example those disclosed in the document PCT / EP2011 / 061244 incorporated by reference in this application.

Finally, it is likely to further optimize the behavior of the system by having an anisochronism of the second oscillator that is not linear. By way of example, the second oscillator may comprise a weak anisochronism around the equilibrium amplitude and a strong anisochronism far from the amplitude of equilibrium, or vice versa.

Claims (14)

Timepiece (1) comprising a first oscillator (15) oscillating at a first frequency ( f ₁ ) and connected by a first gear (5) to a power source (9) to display the time and a chronograph system (51) having a second gear (25) connected to the first gear (5) via a clutch device (44) for selectively measuring a time characterized in that the chronograph system (51) further comprises a second oscillator (35). ) connected to the second gear (25) which oscillates at a second frequency ( f ₂ ) and in that the second gear (25) is connected to the first gear (5) by elastic coupling means (41) to synchronize the gait of the two oscillators (15, 35) using the same power source (9) when the clutch device (44) allows said measurement of a time. Timepiece (1) according to the preceding claim, characterized in that the elastic coupling means (41) are formed by a spring (43) connecting a wheel of the first gear (5) with another of the second gear (25) . Timepiece (1) according to the preceding claim, characterized in that the elastic coupling means (41) connect the wheels of the second respectively of the first gear (5) and the second gear (25). Timepiece (1) according to one of the preceding claims, characterized in that the first oscillator (15) receives the most torque from the energy source (9). Timepiece (1) according to the preceding claim, characterized in that the first oscillator (15) receives at least 75% of the torque supplied by the energy source (9). Timepiece (1) according to one of the preceding claims, characterized in that the first oscillator (15) has a isochronism better than the second oscillator (35) to facilitate the synchronization of the latter. Timepiece (1) according to one of the preceding claims, characterized in that the first oscillator (15) has a quality factor ( Q ₁ ) greater than that ( Q ₂ ) of the second oscillator. Timepiece (1) according to the preceding claim, characterized in that the second oscillator (35) has a quality factor ( Q ₂ ) less than 100 to obtain a faster stabilization. Timepiece (1) according to one of the preceding claims, characterized in that the first ( f ₁ ) and second ( f ₂ ) frequencies are identical. Timepiece (1) according to the preceding claim, characterized in that the two frequencies ( f ₂ , f ₁ ) are greater than 5 Hz to display with a better resolution and / or a better accuracy as well the time as the measured time. Timepiece (1) according to one of claims 1 to 8, characterized in that the first frequency ( f ₁ ) is higher than the second frequency (f ₂ ) to display the time with better resolution and / or better accuracy. Timepiece (1) according to the preceding claim, characterized in that the first frequency ( f ₁ ) is at least 10 Hz and the second frequency ( f ₂ ) between 1 and 5 Hz. Timepiece (1) according to one of claims 1 to 8, characterized in that the first frequency ( f ₁ ) is lower than the second frequency ( f ₂ ) to display the measured time with better resolution and / or better accuracy. Timepiece (1) according to the preceding claim, characterized in that the second frequency ( f ₂ ) is at least 10 Hz and the first frequency ( f ₁ ) between 3 and 5 Hz.

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